Liquid hydrogen and liquid oxygen are carried on board the Space Shuttle during its orbital flights. The specially designed tanks consist of an inner sphere supported inside an outer sphere. The inner sphere is fabricated from two precisely formed hemispheres. The hemispheres are dimensionally inspected, fitted into a weld fixture, positioned for welding, inspected and adjusted for alignment or mismatch and then welded together. The outer sphere also consists of two hemispheres. When assembled, the inner sphere is supported inside the outer sphere with the space between the inner and outer spheres being filled with appropriate insulation material and evacuated. The two outer hemispheres are also joined by welding and a girth ring attached which, among other things, facilitates handling of the total pressure vessel assembly. While the match of the two inner hemispheres are visually checked prior to welding, it at times became advisable to recheck the alignment, or possible mismatch, of the two inner hemispheres after total assembly. Disassembling the total structure would have been prohibitive both as to time and cost.
X-ray or radiographic inspections of metal objects is well known in the art. The usual purpose of such inspections is to determine the existence of any abnormalities in the composition of the object which would detract from its utility. An example of such use of X-rays is in the inspection of welds used to join two metal parts such as pipes or in the fabrication of pressure vessels. X-rays of the weld itself permit the determination of the existence of any imperfections in the weld, such as air blisters, which would impair the strength or integrity of the weld. U.S. Pat. No. 3,087,058 is a particular illustration of this technique.
It is an object of this invention to provide a method for accurately determining the alignment of adjoining metal parts, particularly metal parts that have been joined together by a weld or other suitable means and where visual inspection is either impractical or impossible. Other objects will become apparent from the description of the invention.